RF signals propagating through a medium generally experience non-linear phase characteristics, namely, non-linear phase variation with frequency. Without special processing, such a propagated signal will be detected as a degraded signal.
Prior art devices have been satisfactorily employed for years to achieve phase and amplitude equalization. However, they are severely restricted in the number of frequencies that can be handled by the digital electronics circuitry and the speed with which the equalization is activated.
U.S. Pat. No. 4,771,398, issued Sept. 13, 1988, and assigned to the present assignee, utilizes coherent optical processing to perform phase equalization corrections of RF signals by providing equalization paths for a multitude of discrete frequencies in a parallel operation. By virtue of the prior invention, thousands of discrete frequencies may be handled.
The aforementioned invention utilizes a phase-controlled array in the Fourier plane to cancel phase distortion of the propagated signal. The array is comprised of individual components that have their birefringence electrically altered to correspondingly alter the phase of the particular frequency associated with the element. The corrected optical signal then undergoes photoelectric transformation at a photomixer and the result is a phase-equalized correction signal which corresponds to an input signal prior to its propagation-induced phase distortion.
Although the apparatus of the co-pending application operates satisfactorily, at times environmental effects cause problems due to the fact that an optically converted RF signal and optical local oscillator signal are introduced to the apparatus along parallel paths. These paths are subject to different environmental effects due to vibration, temperatures, dust, etc. Accordingly, it would be advantageous to introduce the RF and local oscillator signals along a common optical path to negate the different environmental effects.